New data from NASA’s Cassini spacecraft reveals that Enceladus’s subterranean ocean completely separates its crust from core. But scientists are still unsure why it hasn’t frozen solid.
One of Saturn’s moons, Enceladus, was previously perceived as a remote, icy world with little interest us Earthlings. But Cassini’s satellites and flybys of Saturn have revealed that Enceladus is much more intriguing than we gave it credit for: between its crust and core lies a global ocean of liquid, life-giving water.
This groundbreaking discovery has been years in the making. Cassini has been orbiting Saturn and collecting data since 2004, and it caught the first hint at Enceladus’s watery interior when it spotted icy plumes of saltwater and organic molecules gushing out of fractures in the crust near the south pole. After that, the hunt was on to find an explanation for these geysers and figure out just how geologically active the moon really is.
Astronomers originally thought that the geysers were powered by frictional heat generated by the edges of the fractures, which they call “tiger stripes,” rubbing together. But further analysis revealed that each geyser is associated with a hot spot of geothermal radiation that was too small to result from frictional heating. Data collected by Cassini on the moon’s gravity instead showed that the geysers are fed by a large subterranean sea in the southern hemisphere.
But now, Cassini scientists have revised their original theory and believe that the sea beneath the crust actually spans the entire moon. The discovery came about after they turned their attention to Enceladus’s rotation and orbit. When they pieced together seven years’ worth of images of the surface, they discovered tiny inconsistencies in the moon’s rotation. While very slight, this wobble was still too large to fit a model where the crust and core were frozen together. The only explanation was that a global layer of fluid separates the crust and core.
It turns out that the moon’s slightly irregular shape causes Saturn’s gravitational pull to have different effects on it as it rotates, so it rocks back and forth. But the magnitude of the moon’s wobble, or libration as it is technically termed, had to be the result of an entire moon’s worth of seawater sloshing back and forth beneath the surface. The researchers estimate that the ocean is between 26 and 31 km deep, as opposed to Earth’s relatively shallow 4-km-deep oceans.
This vast amount of seawater on a moon so far from the sun has obviously put astrobiologists in a tizzy — especially after a study earlier this year identified hydrothermal activity at the bottom of the sea. These vents of heat erupting from deep within the core are just like the ones found on Earth that teem with diverse ecosystems’ organisms. Scientists have long suspected that hydrothermal vents may have provided the perfect conditions for life to begin on our planet back in its infancy. If this hydrothermal activity extends across the floor of Enceladus’s sizable ocean, the vents can supply salts, hydrogen, and organic compounds that support life.
The next step is to figure out why Enceladus’s seas haven’t frozen. It doesn’t receive nearly enough sunlight to melt its icy surface, let alone the waters beneath. And despite its geothermal activity, the moon’s core does not produce enough energy to keep the water in a liquid state. The researchers believe that this heat may actually come from Saturn’s gravity exerting tidal forces on its satellite, causing it to accelerate and generate an unforeseen amount of heat in its core.
Enceladus now joins the ranks of other moons hiding oceanic layers beneath their crust: Europa and Ganymede, both of which orbit Jupiter. All of these watery worlds are strong candidates for extraterrestrial life, despite their distance from the sun. NASA has been toying with the idea of sending dedicated missions to each of these moons to figure out, once and for all, whether they can support life.
In the meantime, we can look forward to a few more answers from Cassini’s next flyby of Enceladus on Oct 28, when it will get as close as 49 km to the moon’s surface.